Quick questions on DNA structure: Watson, Crick, Franklin and Wilkins (HSC Biology Module 5)

DNA (deoxyribonucleic acid) is a double helix of two antiparallel polynucleotide strands.

The two strands are held together by hydrogen bonds between complementary base pairs:

The two strands run in opposite directions. One runs 5' to 3'; the other runs 3' to 5'. This antiparallel orientation matters for DNA replication (DNA polymerase only synthesises in the 5' to 3' direction, which produces the leading vs lagging strand distinction).

The two strands twist around each other in a right-handed double helix. One full turn is roughly 3.4 nm and contains about 10 base pairs. The diameter is about 2 nm.

Rosalind Franklin (King's College London). A skilled X-ray crystallographer. In 1952 she produced Photograph 51, an X-ray diffraction image of DNA that clearly showed the helical structure and the regular spacing of the bases. She was on the verge of publishing her own model.

For many decades, the Watson-Crick attribution dominated public understanding, while Franklin's role was understated. The current HSC syllabus explicitly asks students to recognise Franklin's contribution. Top-band answers name her by name and identify Photograph 51 as the key piece of evidence.

A skilled X-ray crystallographer. In 1952 she produced Photograph 51, an X-ray diffraction image of DNA that clearly showed the helical structure and the regular spacing of the bases. She was on the verge of publishing her own model.

Franklin's colleague. Showed Photograph 51 to Watson without Franklin's knowledge or permission. Wilkins shared the 1962 Nobel Prize with Watson and Crick; Franklin had died in 1958 and was not eligible.

Built the first accurate physical model of the DNA double helix in 1953, using Franklin's X-ray data plus Erwin Chargaff's chemical analysis showing $A = T$ and $G = C$. Their paper "Molecular Structure of Nucleic Acids" was published in Nature on 25 April 1953. It is one of the most cited papers in biology.

Showed in the 1940s that in any DNA sample, the amount of A equals the amount of T, and G equals C. These Chargaff's rules were the crucial constraint Watson and Crick used to figure out base pairing.

This is a 1-mark deduction on a 3-mark question. Always name Franklin (and Wilkins for completeness).

A-T, G-C. Not A-G or T-C. A and G are purines (large); T and C are pyrimidines (small).

A-T has 2 bonds; G-C has 3 bonds. This is why G-C-rich DNA is harder to separate (more energy needed to break more hydrogen bonds).

DNA has deoxyribose; RNA has ribose. DNA has thymine; RNA has uracil. DNA is double-stranded; RNA is usually single-stranded.